Popular 2D Barcodes
QR Codes
QR Code is a matrix symbology that consists of a square array of modules with a finder pattern located at three corners. A wide range of symbol sizes is supported along with four levels of error correction, and the symbology is noted for its high data density.
They can contain up to 4296 characters of data. to put this into perspective, that's more text than this page contains.
QR Codes can be easily read off everything from buses to business cards using almost any current cell phone. They can contain URL's, Text, VCard data (Outlook contacts) and more.
DataMatrix
DataMatrix is a two dimensional matrix symbology which is made up of square modules arranged within a finder pattern. Unlike QR Codes, DataMatrix barcode symbols may be square or rectangular.
They can contain up to 2300 characters of data.
Microsoft Tag
The Microsoft Tag Reader works on Windows Mobile, iPhone, Symbian, Blackberry, Android and J2ME phones plus a number of other supported devices.
Unlike QR Codes and DataMatrix, the Microsoft tag can be read at more extreme angles and tends to be smaller in size while containing the same amount of information.
Since this is a Microsoft technology, some users may tend to stay away from it so it may not enjoy the same popularity and acceptance that QR code currently does.